If you buy a natural tree, check the ends of the needles. Rub some of the sprigs between your fingers. If needles fall off, the tree is already dry. If not, it's a good one.
Cut 1 inch or so off the bottom before you put it into the stand, this will allow it to soak up more water and it will last longer. Keep the water level in your tree stand deep enough to cover the cut base of the trunk, so it can continue to absorb water. If the cut end of the base dries out, the tree will dry out very quickly, too.
Try to keep the water level steady. In the first week, a tree in your home will draw up as much as a quart of water per day. Aspirin, sugar, or commercial mixes added to the water do not appear to really help your tree last much longer. Just keep it watered.